Method for forming multi-layer bumps on a substrate

ABSTRACT

A method for forming multi-layer bumps on a substrate includes depositing a first metal powder on the substrate, and selectively melting or reflowing a portion of the first metal powder to form first bumps. A second metal powder is then deposited on the first bumps, and melted to form second bumps on the first bumps. A masking plate is disposed over the substrate to select the portions of the metal powders that are melted and the metal powders are melted via an irradiation beam. The multi-layer bump is formed without the need for any wet chemicals.

BACKGROUND OF THE INVENTION

The present invention relates to a method for forming bumps on asemiconductor chip or printed circuit board (PCB) environment. Moreparticularly, the present invention relates to a method for formingmulti-layer connectors for flip chip bonding using metal powders andlocalized irradiation.

A typical flip chip assembly uses a direct electrical connection of aface-down semiconductor chip onto a substrate or circuit board viaconductive bumps. Generally, a flip chip assembly is made in threestages, i.e., forming bumps on a chip, attaching the bumped chip to aboard or substrate, and filling the space remaining under the bumpedchip with an electrically non-conductive material.

A conductive bump has several functions in a flip chip assembly, suchas, providing an electrical connection between a semiconductor chip anda substrate, and providing a thermally conductive path to carry heatfrom the semiconductor chip to the substrate. The bump also providespart of the mechanical mounting to the substrate and acts as a spacerfor preventing electrical contact between the semiconductor chip andsubstrate conductors.

There are many methods of forming bumps on a wafer substrate. One methodof forming bumps includes forming a photoresist layer having openingsaligned with bond pads on the wafer substrate, applying a solder pastein the openings by screen printing, and then melting or reflowing thesolder paste to form a bump. The openings may be formed by radiating anddeveloping the photoresist.

One problem of this method is that a new photoresist layer is requiredfor processing each piece of wafer substrate. Another problem is theneed for removal of the photoresist layer by chemical solutions, whichgenerates chemical wastes. Yet another problem is that bump standoff(bump height) depends on the thickness of the photoresist mask. Toobtain a higher standoff, a thicker photoresist layer is required.

However, if a low or fine pitch (bump spacing) is required, the maximumpossible thickness of the photoresist layer is limited. In practice, theopenings in the photoresist layer typically have a reverse conicalshape, i.e., the openings taper towards a narrow end at the bond pads.Hence, there is a tradeoff between a high standoff and a low pitch.

Another method of forming bumps involves patterning a photoresist layerapplied to a wafer substrate to form bump sites and electroplating asolder alloy onto the bump sites. The photoresist layer is then removedbefore reflowing the solder alloy to form a sphere. While thiselectroplating method provides a low pitch, one problem is that wetchemicals or plating bath solutions are required. Further, such chemicalprocesses involve hazardous materials and have to be carefullycontrolled.

In view of the foregoing, it would be desirable to have a method forforming bumps that is low cost and does not involve wet chemicals. Inaddition, it would be desirable to have a method that provides highstandoff (bump height) and low or fine pitch (bump spacing).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings. Tofacilitate this description, like reference numerals designate likestructural elements.

FIG. 1 illustrates an enlarged cross-sectional view of a semiconductorwafer in accordance with one embodiment of the present invention.

FIG. 2 illustrates an enlarged cross-sectional view of the semiconductorwafer of FIG. 1 having a first metal powder in accordance with anembodiment of the present invention.

FIG. 3 illustrates an enlarged cross-sectional view of the semiconductorwafer of FIG. 2 during a first irradiation to the first metal powder inaccordance with an embodiment of the present invention.

FIG. 4 illustrates an enlarged cross-sectional view of the semiconductorwafer of FIG. 3 having a second metal powder over first bumps inaccordance with an embodiment of the present invention.

FIG. 5 illustrates an enlarged cross-sectional view of the semiconductorwafer of FIG. 4 during a second irradiation to the second metal powderin accordance with an embodiment of the present invention.

FIG. 6 is an enlarged cross-sectional view of a number of binal-layermetallic bumps formed on bond pads of a semiconductor wafer inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A method for forming multi-layer bumps or connectors on a substrate in asemiconductor chip or printed circuit board (PCB) environment isprovided. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be understood, however, to one skilled in the art,that the present invention may be practiced without some or all of thesespecific details. In other instances, well known process operations havenot been described in detail to not unnecessarily obscure the presentinvention.

Referring now to FIG. 1, an enlarged, cross-sectional view of asemiconductor chip or wafer or PCB substrate 104 in accordance with oneembodiment of the present invention is shown. The substrate 104 includesa number of bond pads 108 for defining bump sites 112 on which bumps maybe formed. Before forming the bumps, the substrate 104 is cleaned toremove contaminants, such as aluminum oxide, from the bond pads 108.

To accomplish such cleaning, a masking plate 116 patterned with one ormore apertures 120 is disposed over the substrate 104 such that theapertures 120 are aligned with the bump sites 112. A localizedirradiation beam 124, such as, infrared or laser beam is provided overthe masking plate 116 and directed at the bump sites 112. The beam 124burns out any contaminants on the pads 108.

The apertures 120 allow the irradiation beam to pass through to the bumpsites 112 while the masking plate 116 blocks the beam from irradiatingthe rest of the substrate 104. The masking plate 116 may be made ofmetal or ceramic materials, and may have a thickness of about 500microns to about 1 millimeter. The apertures 120 may have diameters fromabout 40 microns to about 60 microns, to closely match the size of thebond pads 108.

Referring now to FIG. 2, a cross-sectional view of the substrate 104having a first metal powder 128 is shown. The first metal powder 128 isdeposited over the substrate 104 to form a substantially uniform layerover the bump sites 112. A masking plate 132 with apertures 136 isdisposed over the substrate 104 such that the apertures 136 on themasking plate 132 are aligned with the bump sites 112. The masking plate132 can be the same as the masking plate 116 used to regulate theirradiation beam 124 as described in FIG. 1.

The first metal powder 128 preferably comprises copper or high leadsolder and has a particle size of about 5 microns to about 10 microns.Though other particle sizes may also be used, it should be appreciatedthat larger particle sizes may result in larger bump sizes and bumppitch. Typically, but not limited to such, the metal powder chosen asthe first metal powder 128 has a melting point of at least about 300degrees Celsius.

Referring now to FIG. 3, a cross-sectional view of the substrate 104during a first irradiation of the first metal powder 128 is shown. Afirst irradiation beam 140 is fired through the masking plate (132 or116), which directs the beam 140 at selected portions of the first metalpowder 128 through the apertures (136 or 120). The selected portions ofthe first metal powder 128 are thus melted or reflowed to form a numberof first bumps 150 on the bond pads 108. The first irradiation beam 140may be any type of beam suitable for heating and melting the first metalpowder, such as an infrared beam or a laser beam. At present, a laserbeam is preferred because it is easy to focus.

Referring now to FIG. 4, a cross-sectional view of the substrate 104having a second metal powder 228 disposed over the substrate 104 and thefirst bumps 150 is shown. The second metal powder 228, preferably havinga lower melting point than the first metal powder 128, is deposited overthe first bumps 150 such as by sprinkling. Typically, but not limited tosuch, the melting point for the second metal powder 228 may rangebetween about 150 degrees Celsius to about 200 degrees Celsius.

The second metal powder 228 may be a eutectic solder (tin-lead, forexample) having a particle size of about 5 microns to about 10 microns,however, it should be appreciated that a larger particle size may resultin larger bump size and bump pitch. A masking plate 232 is disposed overthe second metal powder 228 such that apertures 236 in the masking plate232 are aligned with the first bumps 150 upon which second bumps 250 areto be formed. The masking plate 232 can be the same as the masking plate116 as described in FIG. 1, or the masking plate 132 as described inFIG. 2, or both.

Referring now to FIG. 5, a cross-sectional view of the substrate 104during a second irradiation of the second metal powder 228 is shown. Asecond irradiation beam 240 is fired through the masking plate (232, 132or 116), which directs the irradiation beam 240 at selected portions ofthe second metal powder 228 through the apertures (236, 136 or 120). Theselected portions of the second metal powder 228 are thus melted orreflowed to form a number of second bumps 250 on the first bumps 150.Because the second metal powder 228 has a lower melting point than thefirst metal powder 128, the first bumps 150 do not melt when the secondmetal powder 228 is melted or reflowed to form the second set of bumps250.

The second irradiation beam 240 may be an infrared beam or a laser beam,which heats the second metal powder 228 to a stage at which it issufficiently molten to bond with the first bumps 150. The second bumps250 are then cooled and allowed to solidify. Finally, the unmeltedportions of the first and second metal powders 128 a and 228 a areremoved by, for example, air-blowing or spinning.

In another embodiment of the present invention, bumps may be formed on apad metallurgy, which is provided on the bond pads 108. The padmetallurgy, also known as under-bump metallization (UBM), protects thesubstrate 104 and provides an electrical and mechanical connectionbetween the bumps and an external substrate, such as a printed circuitboard (PCB). The UBM generally comprises successive layers of metalformed on bond pads 108 by methods known to a person skilled in the art.

In another embodiment, the irradiation beam for melting or reflowing themetal powders (128, 228) and for cleaning bump sites 112 described abovemay be replaced with a programmable single laser beam. With theprogrammable single laser beam, heat for melting the metal powders (128,228) can be more precisely directed at the bump sites 112. Hence,portions of the metal powders (128, 228) for forming the bumps (150,250) can be selectively melted without necessarily requiring a maskingplate to regulate heat exposure.

Referring now to FIG. 6, a cross-sectional view of a number ofbinal-layer metallic bumps 350 formed on the bond pads 108 of thesubstrate 104 in accordance with one embodiment of the present inventionis shown. Each binal-layer bump 350 includes a first bump 150 coupled tothe bond pad 108, and a second bump 250 formed upon and coupled to thefirst bump 150. In a flip chip assembly, for example, the binal-layerbumps 350 provide connectors for electrically connecting thesemiconductor substrate 104 to an external substrate in an electronicpackage. Generally, the first bump 150 provides standoff while thesecond bump 250 provides solder joint formation.

While the above process is described in relation to forming bumps on asubstrate 104, the present invention is applicable to forminginterconnects or bumps on PCB substrates. The above process is alsoapplicable to forming a connector having more than the two layers ofbumps. For example, a third bump of the connector can be formed bydepositing a third metal powder over the second bump 250, andselectively melting or reflowing a portion of the third metal powder.

The present invention is particularly advantageous to reduce processingcosts since it requires minimal tooling, involves no wet chemicalprocesses, and utilizes a reusable masking plate. The masking plate maybe eliminated if a programmable, single laser beam is used toselectively melt the metal powders.

Another advantage of the present invention is the high standoff that canbe achieved by binal or multi-layer bumps as compared with single-layerbumps. At high temperatures, the silicon wafer and bumps are subject tothermal mechanical stress caused by different expansion rates in thesilicon wafer and an external surface, such as PCB. The differing ratesof expansion are due to coefficients of thermal expansion (CTE) mismatchin the different materials. Excessive stress may cause silicon fractureor bump fracture. A high standoff releases the stress caused by CTEmismatch and thereby improves bump joint reliability.

A further advantage of the present invention is reduced bump size andbump pitch. By forming the second bump 250 on the first bump 150, a highstandoff is achieved without increasing bump size or diameter. This, inturn, allows a lower or finer bump pitch ranging from about 50 micronsto about 75 microns depending on the metal powder particle size used andresolution of the apertures of the masking plate. In the embodimentwhere programmable laser beam is used, bump size and pitch depend on theresolution of the laser beam.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention. Furthermore, certain terminology has been used for thepurposes of descriptive clarity, and not to limit the present invention.The embodiments and preferred features described above should beconsidered exemplary, with the invention being defined by the appendedclaims.

1. A method for forming a binal-layer bump on a substrate, comprising:depositing a first metal powder on the substrate; melting the firstmetal powder to form a first bump; depositing a second metal powder overat least the first bump; and melting the second metal powder to form asecond bump on the first bump, wherein the first bump and the secondbump form the binal-layer bump.
 2. The method for forming a binal-layerbump according to claim 1, wherein melting the first metal powderfurther comprises: placing a first masking plate over the substrate, thefirst masking plate having at least one aperture; and melting a selectedportion of the first metal powder by irradiating the selected portionthrough the at least one aperture.
 3. The method for forming abinal-layer bump according to claim 2, wherein melting the second metalpowder further comprises: placing a second masking plate over thesubstrate, the second masking plate having at least one aperture; andmelting a selected portion of the second metal powder by irradiating theselected portion through the aperture of the second masking plate. 4.The method for forming a binal-layer bump according to claim 3, whereina melting point of the second metal powder is lower than a melting pointof the first metal powder.
 5. The method for forming a binal-layer bumpaccording to claim 1, wherein the first metal powder and the secondmetal powder each have a particle size between about 5 microns to about10 microns.
 6. The method for forming a binal-layer bump according toclaim 5, wherein the first metal powder comprises one of copper and highlead solder and the second metal powder comprises a eutectic solder. 7.The method for forming a binal-layer bump according to claim 1, furthercomprising cleaning a bond pad on the substrate before depositing thefirst metal powder.
 8. The method for forming a binal-layer bumpaccording to claim 7, wherein the cleaning step comprises irradiatingthe bond pad to burn out any contaminants thereon.
 9. The method forforming a binal-layer bump according to claim 1, wherein melting thefirst metal powder further comprises irradiating a portion of the firstmetal powder with a programmable single laser beam.
 10. The method forforming a binal-layer bump according to claim 1, further comprisingremoving any unmelted portions of the first and second metal powdersremaining on the substrate.
 11. A semiconductor device, comprising: asubstrate having a bond pad; and a binal-layer bump formed on the bondpad, wherein the binal-layer bump includes a first bump coupled to thebond pad and a second bump coupled to the first bump, wherein the secondbump is formed of a material having a lower melting point than amaterial forming the first bump.
 12. The semiconductor device accordingto claim 10, wherein the first bump is formed by melting a first metalpowder deposited on the substrate, and the second bump is formed bymelting a second metal powder deposited over the first bump.
 13. Thesemiconductor device according to claim 12, wherein the first and secondmetal powders each have a particle size between about 5 microns to about10 microns.
 14. The semiconductor device according to claim 13, whereinthe first bump comprises one of copper and high lead solder and thesecond bump comprises a eutectic solder.
 15. The semiconductor deviceaccording to claim 13, wherein the first metal powder has a meltingpoint of above about 300° C.
 16. The semiconductor device according toclaim 15, wherein the second metal powder has a melting point of betweenabout 150° C. to about 200° C.
 17. A method for forming a multi-layerconnector on a substrate, comprising: depositing a first metal powder onthe substrate; irradiating a selected portion of the first metal powderto form a first bump; depositing a second metal powder over the firstbump; and irradiating the second metal powder to form a second bump onthe first bump, wherein the first bump and the second bump form themulti-layer connector.
 18. The method forming a multi-layer connectoraccording to claim 17, wherein the first and second metal powders areirradiated with an irradiation beam directed through an aperture of amasking plate disposed over the substrate.
 19. The method forming amulti-layer connector according to claim 17, wherein a melting point ofthe second metal powder is lower than a melting point of the first metalpowder.
 20. The method forming a multi-layer connector according toclaim 17, wherein the first metal powder and the second metal powdereach have a particle size between about 5 microns to about 10 microns.